Changes so that the keys to the map are used to reference objects in them.  Also add forcedFlags map.
Add forcedFlags map to subpicture.
Set isForced to false initially.
Add flag for when forcedOnly is set.
Pass through flag for forced subtitles export only.
Add better support for exporting just forced subs.  Adds support to remove part of the composition object that is not forced.
setForcedFlags map when setForced is called.